This application generally relates to wireless code division multiple access communication systems. In particular, this application relates to code allocation in such systems using cross code correlation characteristics.
To send multiple communications simultaneously over a shared spectrum, CDMA communication systems distinguish the communications using codes. FIG. 1 illustrates a simplified wireless code division multiple access (CDMA) communication system. The following discussion refers to components that may be located in either the base station, a Node-B or a radio network controller (RNC) 20, based on the implementation, as being located at the base station 20.
For downlink communications, codes are assigned by a code assignment device 26 of the base station 20 to support the downlink communications. Downlink data is processed by a modulation and spreading device 28 with the assigned codes to produce one or multiple downlink communications. The downlink communications are passed through an isolator or switch 32 and radiated by an antenna or antenna array 34 of the base station 20 through a wireless radio interface 46.
The user equipment 22 receives the downlink communications using an antenna or antenna array 36. The received communications pass through an isolator or switch 38 to a data detection device 42. The data detection device 42 recovers data from the received communications using the assigned downlink codes. These assigned codes may be determined by a form of blind code detection, be known a priori, the code assignment may be signaled or by other means. A code assignment determination device 40 determines the assigned codes.
For uplink communications, uplink data is processed by a modulation and spreading device 44 with uplink assigned codes to produce one or multiple uplink communications. The code assignment determination device 40 determines the codes to use for the uplink. The uplink communications are passed through the isolator or switch 38 and radiated by an antenna or antenna array 36 of the UE 22 through the wireless radio interface 46.
The base station 20 receives the uplink communications using its antenna or antenna array 34. The received communications pass through an isolator or switch 32 to a data detection device 30. The data detection device 30 recovers data from the received communications using the assigned uplink codes as determined by the code assignment device 26.
When a user in such a system requires additional codes to support a new or existing service, the code assignment decision is based on the available codes and power budgets. To illustrate, the user requires one additional uplink code to support a new service. A code out of the available codes is selected to support the service. The required transmission power level for a communication using this new code is determined, such as by using a path loss estimate, known interference levels and other factors. If the use of this code requires transmission power levels beyond the systems resources. The new service is not permitted.
As the number of codes increases, the effectiveness of this assignment approach degrades. The likelihood of using or finding codes with good cross correlation properties decreases, as the number of used codes increases. Although codes are designed to have minimal or low cross interference, in practice, some codes may exhibit high cross correlation with other codes. These codes will typically be eliminated from use in the system. Additionally, a group of codes, a code group, may exhibit a high cross correlation with each other, but acceptable cross correlation with other codes. Although eliminating these codes reduces cross interference, it also reduces the number of available codes to assign.
Accordingly, it is desirable to have other code assignment approaches to allow more efficient code assignments.